Rubber is often compounded with silica to provide desired compound properties for tires and other rubber products. The incorporation of the silica into the rubber using a dry mixing process, where a coupling agent is attached to the silica surface during the mixing process to allow it to blend into the rubber, is well known. When the silica is coated with the coupling agent, the silica is referred to as “hydrophobated” or “functionalized”. A silane is often used as the coupling agent for the silica to be incorporated into the rubber.
The reaction of the silica with the silane coupling agent during the dry mixing process undesirably results in a generation of ethanol, which must then be removed during processing. Manufacturing processes for dry mixing simultaneously disperse and distribute silica into the rubber matrix, while removing the ethanol. These processes have been effective but are time-consuming, energy-intensive, and require great amounts of operating and maintenance expenses.
It is also well known to pretreat silica with the coupling agent, prior to incorporating the silica into the rubber compound, as an alternative to the dry mixing process. There are advantages to using a pretreated silica, including low or no alcohol emissions from the silane during mixing, and the elimination of the reaction step during mixing, which is otherwise required when dry mixing. The pretreating of silica is described in various patents including U.S. Pat. No. 3,227,675 to Papalos, U.S. Pat. No. 4,076,550 to Thurn et al., and U.S. Pat. No. 4,151,154 to Berger.
Silica fillers do not disperse easily in rubber using conventional mixing. Especially with high surface area silica, mixing sufficient to provide optimal physical properties has proven difficult due to the inability to disperse the high surface area silica. Achieving good dispersion with high surface area silica using conventional rubber mixing has proven highly problematic or impossible. Thus, the full potential of high surface area silica has not been realized in the tire industry.
High surface area silica is defined herein as silica having a BET specific surface area greater than about 200 m2/g. In a most particular example, the high surface area silica has a BET specific surface area greater than about 220 m2/g. Although pretreated silica is hydrophobated, the pretreated silica typically does not disperse into the rubber as well as other conventional fillers, such as carbon black.
A further alternative to both dry mixing and pretreating silica is the incorporation of silica in a rubber masterbatch in a concentrated amount. The silica-rubber masterbatch, normally provided in a bale, can then be conventionally mixed with the rubber to disperse the silica to a desired loading in the rubber compound. It should be appreciated that there are advantages to mixing of hydrophobated silica in a rubber latex prior to forming the rubber masterbatch, compared to mixing of silica directly into rubber compounds. The silica-rubber masterbatch is typically a combination of silica and polymer and, optionally, other compounding ingredients such as process oil and inert materials. Known silica-rubber masterbatches are described in U.S. Pat. No. 8,357,733 to Wallen et al. and U.S. Pat. No. 5,763,388 to Lightsey et al.
Due to minimal manufacturing complexity, silica-rubber masterbatches most often use an emulsion polymer such as emulsion styrene-butadiene rubber (ESBR) as the masterbatch rubber. However, many rubber compounds, and especially tire rubber compounds, require the use of solution polymers such as solution styrene-butadiene rubber (SSBR) instead of emulsion polymers, for reasons of performance. The use of emulsion polymer-based silica-rubber masterbaches in solution polymer-based rubber compounds can detrimentally affect rubber compound performance, and is therefore undesirable. The use of silica-rubber masterbatches having other types of non-solution polymers such as natural rubber is also undesirable.
There is a continuing need for a functionalized silica and polymer composite that can be incorporated into a rubber system with excellent dispersion and minimal dust release. Desirably, the functionalized silica and polymer composite may be incorporated into a predominantly SSBR rubber formulation without significantly affecting overall performance in an end product.